This invention relates to objective lenses that contain aspherical surfaces and are designed to be manufactured in high volumes.
More particularly, the invention relates to an inverse triplet objective lens which:
(1) has a negative (or weak positive), positive, negative configuration;
(2) uses aspherical surfaces to correct primary aberrations; and
(3) uses aspherical surfaces to reduce manufacturing sensitivities by reducing the amount of aberration correction contribution by an individual lens element.
In certain embodiments, the negative (or weak positive), positive, negative configuration is followed by a positive lens unit which serves to provide the lens with an extended exit pupil.
It is well know that the classical triplet (see FIG. 6A and Table 6) can be corrected for all of the primary aberrations, but because there is a residual secondary astigmatism that is not correctable, the performance is limited by astigmatism. If the specifications for the lens are xe2x80x9cpushedxe2x80x9d too far, the astigmatism is so severe that the depth of focus becomes very shallow (see FIG. 6B), and the lens becomes very difficult to manufacture simply because any manufacturing variations will further reduce this already shallow depth.
A further disadvantage of the triplet design is that the spherical aberration and astigmatism correction are both achieved because the negative element introduces just the xe2x80x9crightxe2x80x9d amount of aberration of the opposite sign to cancel the effects of the two positive elements. This means that strongly aberrated lenses must be located properly, without tilts or decentrations, or the aberrations will not completely cancel each other, as demonstrated by the through-focus MTF for a decentered second element shown in FIG. 6C.
There are many four element lens types that are much better than the classical triplet and are usually employed to achieve higher performance than the triplet, but with some increase in cost. Even employing aspherical surfaces only results in limited improvement of a classical triplet because the stop is located too close to the negative element for the astigmatism to be corrected and the manufacturing sensitivity problem still remains.
In view of the foregoing, it is an object of the invention to provide improved objective lenses. More particularly, it is an object of the invention to provide objective lenses having improved aberration correction and reduced sensitivity to manufacturing variations. It is a further object of the invention to provide objective lenses which use a minimum of lens elements, e.g., three or four elements, where, preferably, all of the lens elements are composed of moldable materials and have aspherical surfaces.
To achieve the foregoing and other objects, the invention in accordance with certain of its aspects provides an optical system for producing an image of an object, said system having an overall positive optical power xe2x80x9c"PHgr"0xe2x80x9d, an overall positive focal length xe2x80x9cf0xe2x80x9d(f0=1/"PHgr"0), an object side, and an image side, and comprising in order from its object side to its image side of:
(a) a first lens unit (U1) having a negative power or a weak positive power (i.e., a positive power which is less than 0.3 times the overall positive power of the optical system and preferably is less than 0.2 times the overall positive power) and an object side surface and an image side surface;
(b) a second lens unit (U2) having a positive power and an object side surface and an image side surface;
(c) a third lens unit (U3) having a negative power, the magnitude of the focal length of the third lens unit being less than 1.5 times the overall positive focal length of the optical system;
(d) an aperture stop (AS) between the object side surface of the first lens unit and the image side surface of the second lens unit; and
(e) at least one aspherical surface.
In certain preferred embodiments, the optical system has some or all of the following characteristics, either separately or in combination:
(1) each of the first, second, and third lens units comprises at least one aspherical surface;
(2) at least two of the first, second, and third lens units comprises two aspherical surfaces;
(3) each of the first, second, and third lens units comprises two aspherical surfaces;
(4) each of the first, second, and third lens units consists of a single lens element;
(5) the operative imaging components of the optical system, i.e., the components with optical power, consist of just the first, second, and third lens units;
(6) the operative imaging components of the optical system, i.e., the components with optical power, consist of just the first, second, and third lens units and each of those units consists of a single lens element;
(7) the optical system comprises a fourth lens unit (U4) on the image side of the third lens unit, said fourth lens unit having a positive power;
(8) the fourth lens unit comprises at least one aspherical surface;
(9) each of the first, second, third, and fourth lens units consists of a single lens element;
(10) the operative imaging components of the optical system, i.e., the components with optical power, consist of just the first, second, third, and fourth lens units;
(11) the operative imaging components of the optical system, i.e., the components with optical power, consist of just the first, second, third, and fourth lens units and each of those units consists of a single lens element;
(12) all lens elements used in the optical system are made from moldable materials, e.g., plastics or moldable glasses;
(13) the back focal length of the optical system is at least 0.5 times the overall positive focal length of the optical system;
(14) the exit pupil of the optical system is located at a distance from the image of at least 2 times the overall positive focal length of the optical system;
(15) the distance from the object side of the first lens unit to the image is less than 2 times the overall positive focal length of the optical system;
(16) at least one surface of the lens element nearest the image has an inflection;
(17) the optical system has a half field of view in the direction of the object of at least 25xc2x0;
(18) the optical system has a relative aperture of less than f/5, e.g., a relative aperture of f/4 or f/3.5;
(19) the optical system is used as a taking lens for a digital light sensor; and
(20) the optical system comprises sufficient aspherical surfaces to substantially correct third and fifth order aberrations and to substantially reduce the system""s sensitivity to manufacturing deviations (tolerances).